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Property, plant and equipment (Details 3) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Property, plant and equipment [Line Items] |  |  |  |  |  |
Interest capitalized | $ 6,033 |  | $ 8,659 |  | $ 10,821 |
AFUDC - borrowed | 2,767 |  | 2,483 |  | 1,666 |
AFUDC - equity | 3,322 |  | 4,530 |  | 2,587 |
Property, plant and equipment | 8,803,866 |  | 8,107,751 |  |  |
Less accumulated depreciation, depletion and amortization | 3,872,487 |  | 3,608,912 |  | 3,361,208 |
Net property, plant and equipment | 4,931,379 |  | 4,498,839 |  | 4,285,014 |
BOE average rate for amortization under units-of-production method (in dollars per BOE) | 17.41 |  | 15.28 |  | 12.25 |
Oil and natural gas properties not subject to amortization | 124,869 |  | 191,794 |  | 232,462 |
Electric: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,315,822 | [1] | 1,150,584 | [1] | 1,068,524 | [1]
Natural gas distribution: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,776,901 | [1] | 1,689,950 | [1] | 1,568,866 | [1]
Pipeline and energy services: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 962,172 |  | 816,533 |  | 719,291 |
Exploration and production: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 3,060,848 |  | 2,764,560 |  | 2,615,146 |
Construction materials and contracting: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,510,355 |  | 1,504,981 |  | 1,499,852 |
Construction services: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 134,948 |  | 130,624 |  | 124,796 |
Other: |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 49,997 |  | 50,519 |  | 49,747 |
Regulated: | Electric: | Generation |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 570,394 |  | 546,011 |  |  |
Weighted average depreciable life in years | 42 years |  |  |  |  |
Regulated: | Electric: | Distribution |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 308,202 |  | 276,446 |  |  |
Weighted average depreciable life in years | 39 years |  |  |  |  |
Regulated: | Electric: | Transmission |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 196,824 |  | 180,543 |  |  |
Weighted average depreciable life in years | 48 years |  |  |  |  |
Regulated: | Electric: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 141,365 |  | 62,123 |  |  |
Regulated: | Electric: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 99,037 |  | 85,461 |  |  |
Weighted average depreciable life in years | 14 years |  |  |  |  |
Regulated: | Natural gas distribution: | Distribution |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 1,384,587 |  | 1,308,314 |  |  |
Weighted average depreciable life in years | 40 years |  |  |  |  |
Regulated: | Natural gas distribution: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 46,763 |  | 71,679 |  |  |
Regulated: | Natural gas distribution: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 345,551 |  | 309,957 |  |  |
Weighted average depreciable life in years | 25 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Transmission |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 418,594 |  | 403,126 |  |  |
Weighted average depreciable life in years | 52 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Gathering |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 39,597 |  | 42,420 |  |  |
Weighted average depreciable life in years | 19 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Storage |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 42,939 |  | 42,058 |  |  |
Weighted average depreciable life in years | 51 years |  |  |  |  |
Regulated: | Pipeline and energy services: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 6,937 |  | 13,667 |  |  |
Regulated: | Pipeline and energy services: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 39,504 |  | 38,386 |  |  |
Weighted average depreciable life in years | 29 years |  |  |  |  |
Nonregulated: | Eliminations |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | (7,177) |  | 0 |  |  |
Nonregulated: | Pipeline and energy services: | Midstream |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 213,063 |  | 233,840 |  |  |
Weighted average depreciable life in years | 17 years |  |  |  |  |
Nonregulated: | Pipeline and energy services: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 188,641 |  | 29,657 |  |  |
Nonregulated: | Pipeline and energy services: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 12,897 |  | 13,379 |  |  |
Weighted average depreciable life in years | 11 years |  |  |  |  |
Nonregulated: | Exploration and production: | Oil and natural gas properties |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 3,017,879 | [2] | 2,723,356 | [2] |  |
Nonregulated: | Exploration and production: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 42,969 |  | 41,204 |  |  |
Weighted average depreciable life in years | 8 years |  |  |  |  |
Nonregulated: | Construction materials and contracting: | Land |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 125,551 |  | 126,788 |  |  |
Nonregulated: | Construction materials and contracting: | Buildings and improvements |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 70,000 |  | 73,884 |  |  |
Weighted average depreciable life in years | 19 years |  |  |  |  |
Nonregulated: | Construction materials and contracting: | Machinery, vehicles and equipment |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 906,774 |  | 899,592 |  |  |
Weighted average depreciable life in years | 12 years |  |  |  |  |
Nonregulated: | Construction materials and contracting: | Construction in progress |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 13,315 |  | 11,165 |  |  |
Nonregulated: | Construction materials and contracting: | Aggregate reserves |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 394,715 | [3] | 393,552 | [3] |  |
Nonregulated: | Construction services: | Land |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 4,821 |  | 4,723 |  |  |
Nonregulated: | Construction services: | Buildings and improvements |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 16,628 |  | 16,563 |  |  |
Weighted average depreciable life in years | 20 years |  |  |  |  |
Nonregulated: | Construction services: | Machinery, vehicles and equipment |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 105,991 |  | 100,445 |  |  |
Weighted average depreciable life in years | 6 years |  |  |  |  |
Nonregulated: | Construction services: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 7,508 |  | 8,893 |  |  |
Weighted average depreciable life in years | 4 years |  |  |  |  |
Nonregulated: | Other: | Land |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | 2,837 |  | 2,837 |  |  |
Nonregulated: | Other: | Other |  |  |  |  |  |
Property, plant and equipment [Line Items] |  |  |  |  |  |
Property, plant and equipment | $ 47,160 |  | $ 47,682 |  |  |
Weighted average depreciable life in years | 23 years |  |  |  |  |
[1] | Includes allocations of common utility property.
[2] | Amortized on the units-of-production method based on total proved reserves at a BOE average rate of $17.41, $15.28 and $12.25 for the years ended December 31, 2013, 2012 and 2011, respectively. Includes oil and natural gas properties accounted for under the full-cost method, of which $124.9 million and $191.8 million were excluded from amortization at December 31, 2013 and 2012, respectively.
[3] | Depleted on the units-of-production method.

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- Definition Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.14) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 14 -Article 5 + Details
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